Remove the bars that are not staying down by grabbing each end of the xylophone bar and pulling straight up in the air. Do not tilt the bar while removing it from the instrument since you can easily break the small rubber pin that holds the bar in place.
Check to make sure that none of the braces have come loose beneath the xylophone bars. If any of the guides have moved, gently move them back into position and then replace the bar. Ensure that you put the bar in between the guides without bending them.
Play the xylophone by hitting the middle of the bar. If you hit the bottom of the bar, the xylophone bars will pop up while you are playing. Keep your mallets focused in the center.